A suffragan diocese is a diocese in some Christian Churches, in particular the Catholic and Coptic Churches, that is overseen not only by its own diocesan bishop but also by a metropolitan bishop. The metropolitan is always an archbishop who governs his own archdiocese. He has no canonical right of intervention in a suffragan see but rather presides at meetings of the ecclesiastical province composed of his diocese and the suffragan dioceses. It also falls to the metropolitan to run a suffragan diocese when it has no bishop. The bishop of a suffragan diocese is called a suffragan bishop.
Source: "Catholic Encyclopedia"